1. Field of the Invention
The present invention relates to a system and method for producing salts in a crystallizer having an integrated column and, more particularly, for producing sodium metabisulfite.
2. Description of the Related Art
Various additives can be added to food and beverage products to preserve them from spoilage. For example, the use of sulfur dioxide (SO2) in winemaking dates back several thousand years. In earlier times, sulfur dioxide was introduced into wine by burning elemental sulfur in a container intended to contain wine. As wine was introduced to the container, the sulfur dioxide would dissolve in the wine as sulfite and protect the wine from oxidation. Today, pure salts of sulfur dioxide may be used. One such salt is sodium metabisulfite (Na2S2O5), also referred to as sodium pyrosulfite. Sodium metabisulfite has multiple alternative uses, for example, as a photographic developer, in chrome tanning of leather, in the manufacture of dyes and chemicals, as a bleach, a reducing agent for emulsion, rayon and paper and in the medical industries.
Salts, such as sodium metabisulfite, are typically produced by precipitation reactions and generally crystallize to form an orderly and repetitive arrangement of salt molecules. Crystal formation first involves nucleation of a solid phase from a liquid phase followed by crystal growth, the orderly addition of the molecules. Supersaturation is the driving force effecting this phase change.
Salt crystals have been made using several systems and methods. Various methods of crystallization are discussed in Perry's Chemical Engineers' Handbook, 7th ed. (1997) edited by Robert H. Perry, Don W. Green and James O. Maloney. Batch crystallization, for example, involves the charging of a concentrated or near-saturated solution followed by a time controlled cooling or evaporation of the solvent. In some processes, the resulting solution can then be artificially seeded. At the end of the cycle, the slurry may be filtered or dried to recover the crystals.
Another method involves the use of a continuous forced-circulation evaporative crystallizer. The process includes introducing a feed salt solution to the body of an evaporative crystallizer. A slurry solution is withdrawn from the body of the crystallizer and heated, typically with a heat exchanger or heating element. The heated slurry is returned to the body and mixes with the slurry body and raises the slurry body temperature locally near the point of entry causing boiling at the liquid surface. The consequent cooling and vaporization results in increasing saturation and promotes crystal precipitation in the crystallizer.
A crystallizer may use mechanical circulation or agitation. In an agitated crystallizer, an impeller agitates the slurry to promote nucleation and growth. Crystals eventually settle to the bottom of the crystallizer body in the settling zone. Above the settling zone is a region of supernatant liquid relatively free of crystals. A circulation pump withdraws supernatant liquid and feeds it to a heating element sufficiently sized to compensate for evaporative cooling.
Reactive crystallizers take advantage of the phase change created by the product of a chemical reaction. For example, in producing ammonium sulfate crystals, gaseous ammonia is introduced into the reactive crystallizer. Simultaneously, sulfuric acid is introduced at the bottom of the crystallizer body and is mixed with mother liquor where it reacts with the ammonia to produce ammonium sulfate salt. Ammonium sulfate crystals precipitate and settle at the bottom of the crystallizer body.
In Saeman, U.S. Pat. No. 3,960,502, an absorber-crystallizer tower with means for spraying aqueous solution into a reactive gas to saturate the solution from which the product crystallizes is described. The apparatus also has an improved means for removing scale from the crystal containing suspension.
One method of producing sodium metabisulfite disclosed by Metzger et al., U.S. Pat. No. 3,860,695, includes adding sodium hydroxide to a gas-free reaction medium. Flue gas, containing sulfur dioxide, is contacted with a reaction medium using a Venturi scrubber whose outlet is submerged in a reaction medium containing sulfur dioxide. This reaction produces sodium metabisulfite. The sodium metabisulfite crystals may be then allowed to settle.
Another method of producing sodium metabisulfite disclosed by Zolotoochin, U.S. Pat. No. 5,976,485, involves introducing sulfur dioxide into a solution of sodium sulfite and sodium bisulfite to produce sodium bisulfite in a solution containing dissolved sulfur dioxide. The solution is transferred to a crystallizer where it is cooled while mixing with an alkali to produce sodium metabisulfite.
In Bean, U.S. Pat. No. 3,995,015, a process for the manufacture of sodium metabisulfite from sulfur dioxide and sodium carbonate is described. The process of Bean involves passing the sulfur dioxide containing gas serially through two reaction media where the first reaction is maintained under strongly acidic conditions and the second reaction is maintained under strongly alkaline conditions. The first reaction medium contains an acidic aqueous solution of sodium metabisulfite. The second reaction medium contains alkaline aqueous slurry of sodium carbonate. Sulfur dioxide gas is introduced in the first reaction medium. Unabsorbed sulfur dioxide gas is introduced into the second aqueous reaction medium to form sodium sulfite. Sodium carbonate is introduced into the second reaction medium to maintain alkalinity. An alkaline aqueous dispersion of sodium carbonate containing sodium sulfite and sodium carbonate from the second reaction medium is transferred to the first reaction medium. A solution from the first reaction medium overflows to the second reaction medium. A solution of sodium metabisulfite containing suspended sodium metabisulfite crystals is withdrawn from the first reaction medium for crystal separation. The crystal-free solution is returned to the first aqueous reaction medium.
In another patent to Bean, U.S. Pat. No. 4,884,880, sodium carbonate and sulfur dioxide gas react in a vessel. The resultant solution then flows into a crystallizer where it is cooled to precipitate sodium metabisulfite.